Thermoplastic resin sheets have been applied to transparent sheet substrates or front panels for display devices, glazing in hospitals, etc. Examples of the thermoplastic resins used in these applications include methacrylic resins (PMMA), polyethylene terephthalate resins (PET), polycarbonate resins (PC) and vinyl chloride resins (PVC). However, PMMA is poor in a heat deformation resistance and high in a water absorption although it is excellent in a surface hardness and weather resistance. PET is insufficient in a surface hardness although it is excellent in an impact resistance. PC is insufficient in a surface hardness, weather resistance and chemical resistance although it is excellent in a heat deformation resistance and impact resistance. PVC is poor in a heat deformation resistance although it is inexpensive and flame-retardant.
Methyl methacrylate/styrene copolymers (MS resins) obtained by modifying methacrylic resins by copolymerization to impart a low water absorption have been widely used as a transparent sheet substrate for display devices. However, MS resins have a low transparency as compared with methacrylic resins because of their aromatic ring structure, to limit their applications in some cases. In addition, MS resins have been required to be further improved in their dimensional stability and heat deformation resistance.
Resins for plastic optical articles such as plastic lenses and light guide plates which are produced by injection molding are required to have a high melt fluidity in addition to the above properties. If the melt fluidity of the resins is high, the resin can reach all the corners of a mold to provide a molded article which precisely reproduces the contour of the cavity.
The fluidity of molten resins may be controlled to some extent by varying the structure, molecular weight or molding temperature. However, when the primary structure of resin is changed to decrease a glass transition temperature, the resultant molded articles fail to have a heat deformation resistance. The fluidity can be improved also by reducing the molecular weight, but limited in view of mechanical properties. The fluidity becomes higher with increasing molding temperature. However, the range of molding temperature is limited because a heat degradation deteriorates the mechanical properties, a discoloration occurs, or the gas generated from volatile components impairs the appearance of molded articles.
In the application field of optical articles, the change of color for the worse due to discoloration is a most important problem. Also, a high heat deformation resistance is required so as to effectively reuse the pieces of scrap by recovery, crushing and re-molding. Hitherto, it has been attempted to improve the heat deformation resistance by adding an additive. However, the use of the additive should be avoided as much as possible in the application field of optical articles to enhance the optical purity. Therefore, it has been required to improve the heat deformation resistance of the resins themselves so as to prevent the discoloration due to the decomposition of resin.
The hydrogenation of the aromatic ring of styrene-based resins (hydrogenation of aromatic double bond) is known for a long time. Polyvinylcyclohexane obtained from polystyrene has an excellent transparency and heat deformation resistance although poor in the mechanical strength. With its excellent transparency and heat deformation resistance, the application of polyvinylcyclohexane to optical disk substrates has been studied (Patent Document 1). Patent Documents 2 and 3 disclose the application of a resin having a specific monomer composition to optical disks or plastic lenses. The resin is obtained by hydrogenating the aromatic double bond of MS resin. Since the proposed resins contain vinylcyclohexane repeating units in an amount of 50% or more of the total repeating units, the adhesion to metal is insufficient and the heat deformation resistance is not necessarily sufficient. Therefore, the properties required for the optical disk substrates are not obtained in some cases. When applied to plastic lenses, the mechanical properties fail to satisfy the practical requirement in some cases.
An example of the optical articles is a backlight-type light guide plate for use in surface light-emitting devices. A light guide plate with a size of 20 inch or smaller is produced by injection molding in many cases. A light guide plate for use in large-sized surface light-emitting devices with a size exceeding 20 inch is produced in many cases by cutting a thermoplastic resin sheet. With increasing size of recent liquid displays, the development of a surface light-emitting device having a uniform light-emitting performance with a high luminance without ununiformity is demanded. Further, with the increasing demand for color displays, the surface light-emitting device is also required to have an excellent color reproducibility and color stability. Recently, the surface light-emitting device is often operated under much severer conditions to improve the luminance, thereby arising considerable problems of the discoloration of the light guide plates due to its degradation and the change in the color of emitted light.
These problems are tried to be solved by the addition of various antioxidants or ultraviolet ray absorbents and the color compensation using additives. However, these methods cause ununiform luminance or color, thereby failing to achieve a precise color representation.
In addition, the increase in the size of display screen requires a low water absorption. When the resin absorbs water, the screen warps and the luminance and color becomes ununiform.
Optical screens such as transmission-type screens for projection televisions are usually constituted from lens units such as a Fresnel lens sheet and a lenticular lens sheet. The Fresnel lens sheet is produced by forming Fresnel lenses on a thermoplastic resin substrate. The lenticular lens sheet is produced by forming lenticular lenses on a thermoplastic resin substrate. The Fresnel lens has a stepwise lens surface in place of a continuous lens surface, and can be regarded as a concentric prism. The lenticular lens is a plate of lens array in which semi-cylindrical lenses are arranged side by side such that the lenses are in axially parallel to one another. As resins for these substrates, acrylic resins have been used in view of their good transparency, such as an acrylic resin added with a multi-layered rubber component (Patent Document 4), a methacrylic resin containing tert-butylcyclohexyl methacrylate units (Patent Document 5), and a methyl methacrylate/styrene copolymer resin containing the copolymerized styrene in an amount of 36% by weight (Patent Document 6).
When a substrate is made of a resin having a high water absorption such as PMMA, the screen is likely to cause a dimensional change. When being made of a resin having a low polarity such as polystyrene, the surface adhesion is poor to likely cause the lenses made of an ultraviolet curing resin to peel off the substrate. As the light-emitting source is recently changed from CRT to a liquid crystal display, a substrate with no birefringence comes to be strongly demanded. To meet such demand, the resin for the substrate is required to be well-balanced in a low water absorption, a low birefringence and a good adhesion to ultraviolet curing resins.
Another example of the optical articles includes a front panel for display devices. Important properties are antireflection, scratch resistance, stain resistance, etc. The front panel should transmit visible light therethrough as uniformly as possible, in addition to being resistant to warp due to the absorption of water.
Optical articles having a plate shape or a complicated structure such as a small-sized light guide plate is mainly produced by injection molding.
Other optical articles produced by injection molding includes, for example, a plastic lens. The material of the plastic lens should reproduce the cavity shape even in a thin-wall portion. To improve the recording density of optical recording media, it is recently studied to reduce the wavelength of laser for recording and reproduction of information, particularly, 350 to 450 nm. Therefore, a lens meeting such requirement is demanded.
Patent Document 7 discloses a vinyl alicyclic hydrocarbon-based resin as a resin suitable for the plastic lens, particularly, as a resin applicable to blue laser near 405 nm. However, the proposed resin is poor in mechanical properties upon use, because the lens cracks at its holding portion during the use, even when molded into a lens with a proper shape.
Still another example of optical articles is a substrate for optical recording media which is mainly made of polycarbonate. However, such substrate comes to face problems in the birefringence and warp, as the capacity of magneto optical recording disks is increased and the recording density is increased as in the case of developing the digital versatile disc or the blue laser diode. To solve these problems, a hydrogenated polystyrene is proposed as a substitute for polycarbonate (Patent Document 8). Another proposed substitute is a hydrogenated styrene/conjugated diene block copolymer in which styrene is block-copolymerized with a conjugated diene such as isoprene and butadiene to introduce a rubber component (Patent Document 9). However, since the haze may increase if the hydrogenation is incomplete, these hydrogenated styrene-based aromatic hydrocarbon polymers are not suitable for the substrate of optical recording media.
Patent Document 1: JP 63-43910A
Patent Document 2: JP 6-25326A
Patent Document 3: JP 4-75001A
Patent Document 4: JP 1-128059A
Patent Document 5: JP 2-254434A
Patent Document 6: JP 9-302176A
Patent Document 7: JP 2001-272501A
Patent Document 8: JP 7-114030B
Patent Document 9: Japanese Patent 2730053